Add a wasmtime-specific wasmtime_wat2wasm C API (#1206)

* Add a wasmtime-specific `wasmtime_wat2wasm` C API

This commit implements a wasmtime-specific C API for converting the text
format to the binary format. An upstream spec issue exists for adding
this to the C API, but in the meantime we can experiment with our own
version of this API and use it in the C# extension, for example!

Closes #1000

* Reorder arguments

* Use wasm_byte_vec_t for input `*.wat`

* Mark wat input as const

* Return an error message and use `fixed`

* Actually include the error message

* Use `fixed` in `Module.cs` as well

crates/c-api/Cargo.toml

@@ -20,3 +20,4 @@ doctest = false
 wasmtime = { path = "../api" }
 wasi-common = { path = "../wasi-common" }
 wasmtime-wasi = { path = "../wasi" }
+wat = "1.0"